1. Technical Field
The embodiments herein generally relate to microscale manufacturing techniques, and, more particularly, to microscale manufacturing techniques for electrical and mechanical connections.
2. Description of the Related Art
The assembly of devices from different materials and/or fabrication processes is a fundamental requirement of any modern system manufacturing procedure. Such assembly is often performed at the device level. For example, a printed circuit board is a system platform which electrically connects a number of individual devices, including resistors, capacitors, inductors, sensors, or integrated circuit (IC) chips. Each of these devices comes from a unique fabrication process, and each device size is constrained in part by the process used to assemble these devices onto the printed circuit board. In addition to the printed circuit board, system platforms include flat or flexible panel displays, bio-compatible medical devices, smart cards, and many other systems which integrate conventional ICs with unconventional substrates.
Modern system integration methods typically use manual or robotic “pick-and-place” techniques. However, below a certain component size scale, these techniques typically cannot handle and deterministically place components with reasonable efficiency and cost. The smallest capacitor package which a modern robotic pick-and-place machine is specified to handle is approximately 300 μm by 600 μm, and suggests that the difficulty of handling even smaller devices would increase beyond a cost-effective level. Modern IC and MEMS manufacturing processes produce devices much smaller than 300 μm, but current assembly methods typically cannot handle such small, individual devices. Therefore, there is a need for cost effective methods of component assembly in the size scale below 300-600 μm.
One method for assembling small devices is to effectively employ similarly-small pick-and-place machines. However, a limitation with such methods is that other forces can overcome the inertial or gravitational forces required to release parts at the desired stage in a pick and place process. These forces contribute to an observed stiction phenomenon. A further limitation is that the deterministic handling of parts becomes infeasible as the number of parts increases, because of the serial nature of such methods.
Self-assembly is an attractive alternative manufacturing paradigm for continued miniaturization and increased functional microsystem integration. A viable self-assembly packaging and integration process requires both assembly and alignment of individual parts to any desired binding sites, as well as electrical and mechanical connections to these binding sites. Furthermore, these processes must be cost-effective, and scalable in terms of numbers of parts assembled and in terms of part size.
One approach to self-assembly is based on magnetic forces where templates are composed of alternating layers of magnetized and non-magnetized material to form a laminated structure with an array of magnetic regions. Components are vibrated and trapped by the magnets to form a matching array. However, a limitation exists with the use of a laminated structure, in that the application of this method to large numbers of assembly sites would be infeasible. Another limitation is that strong magnetic fields remain permanently in the vicinity of each part following assembly, which would affect the electromagnetic characteristics of any device and may be incompatible with many device technologies. A further limitation is that additional process steps are required to form permanent mechanical and electrical connections, which increase cost.
A related approach utilizes magnetized or electrified binding sites using two-dimensional, planar fabrication methods. Such an approach overcomes the limitation associated with laminated structures, and through the use of electrically active structures as opposed to passive, permanent electrets or magnets, the approach also overcomes the limitation of permanent electromagnetic fields in the vicinity of each part. However, the limitation remains of requiring additional process steps to form permanent mechanical and electrical connections, which increases cost.
Another approach involves fluidic self-assembly where individually shaped micrometer-sized parts are integrated into correspondingly shaped recesses on a substrate using a liquid medium, or carrier fluid, for transport. The key features of the approach are that gravitational and fluidic forces guide parts into the desired wells, and each part has a characteristic trapezoidal shape such that it only fits into its well in the desired orientation. Enhancements to fluidic self-assembly include different-shaped wells for self-selecting different part shapes, and the specific use of a gas as the carrier fluid. A limitation with these methods is only gravity will keep the parts in desired wells following assembly, and therefore the parts may disengage from the wells upon further handling. The same limitation also exists in that post processes are required to form permanent mechanical and electrical connections, which is an example of further handling.
In other approaches related to fluidic-assembly, a template may contain specific electrical conductor patterns, such that when electrically charged by an external source, local electric or magnetic fields help guide and trap parts at the desired locations. Limitations with these approaches include high costs associated with the difficulty of providing temporary electrical connections to addressable electrode structures on the template, where the template is typically submerged in a fluid containing the parts to be assembled. The same limitation also exists in that post processes are required to form permanent mechanical and electrical connections.
One way to overcome some post-processing limitations is to take advantage of surface energy effects, such as hydrophobic or hydrophilic interactions. For example, a substrate can be chemically treated to create regions with a hydrophilic or hydrophobic nature. By also treating parts with complementary hydrophobic or hydrophilic surface treatments, hydrophobic parts can be made to adhere only to a hydrophobic template region, and similar effects can be used with hydrophilic parts and regions. Although any hydrophobic or hydrophilic interactions disappear once the surrounding fluid is removed, hydrophobic and hydrophilic effects may be extended through the use of capillary forces. The effects of capillary forces may be made into permanent mechanical bonds via heating a cross-linkable polymer, or through the contact hardening of cyanoacrylate adhesives. One way to control the assembly at particular template locations is by the use of a carrier fluid which increases its viscosity upon local heating. This viscosity increase can “screen” parts from assembling at particular sites while other sites remain available. However, in all of these approaches, a limitation remains in that further processing is needed to make electrical connections.
An example of further processing is the use of an electrolyte carrier fluid for subsequent electroplating. However, this method requires the attachment of temporary electrical connections to addressable electrode structures on the template, which may increase cost and complexity. Another way to form electrical connections directly as part of a fluidic self-assembly process is to take advantage of capillary forces from a molten alloy or solder. This idea has been explored with millimeter to centimeter scale parts, and more recently with micrometer-scale parts, and in conjunction with the complimentary shaped wells of a fluidic self-assembly process. In each of these examples, a single alloy is used for both mechanical assembly and electrical connections, mandating the use of large electrical contacts and/or alloys which melt at near room temperature. The reason is that comparatively long times are required for stochastic self-assembly processes to reach desirable yields, and high-temperature fluid flux environments cause excessive molten alloy degradation and intermetallic growth when the contact size is too small. The reported electrical conductance per unit area of these contacts is 1.5 to 2.0 mΩ−1 cm−2, which are over three orders of magnitude lower than macroscopically-formed molten alloy contacts, indicating the level to which the contacts can degrade. Accordingly, there is clearly a need to improve the state of the art in molten-alloy based self-assembled electrical contacts.